1. Introduction
This invention relates to a process for pickling metals in acid baths to prepare the surface for a further finishing operation such as metal plating, electrolessly or electrolitically, painting and the like, and more particularly, to pickling metals with hydrohalide acid solutions, particularly hydrochloric acid solutions.
2. Description of the prior art.
Sulfuric acid is the acid which is used in the largest quantities for the pickling of metals because of its low cost. However, the art has recently turned to hydrohalide acids, particularly hydrochloric acid, because of difficulties associated with the disposal of spent sulfuric acid solutions and various advantages associated with the use of the hydrohalide acids such as faster pickling rate at room temperature, better solubility for various types of rust and scale, superior surface conditions of the pickled part and finally, the acid which remains on the surface of the part is both less in volume due to a lower viscosity and more easily removed from the surface of the part by rinsing. The subject of hydrohalide acid pickling of iron and steel is discussed in detail by Max Straschill, Pickling of Metals, Robert Draper, Lmtd. Teddington, England, 1963, pages 42 to 44 incorporated herein by reference. The pickling of other metals with hydrohalide acids is disclosed in the Metal Finishing Guidebook Directory for 1972, Metal and Plastics Publications, Westwood, N.J., pp. 206 to 214, also incorporated herein by reference.
Though the art is turning to hydrohalide acids for pickling, there are certain disadvantages associated with their use. At the concentration at which the acid is used, there is substantial evolution of gas even at room temperature and considerable fuming in moist air due to the formation of hydrohalide acid mist. As well as possessing a sharp odor, these fumes attack the human respiratory organs and mucous membranes and are corrosive to steel and other metal parts with which they come into contact. Moreover, this fuming results in a large loss of acid substantially increasing the cost of the overall pickling procedure. Finally, the excessive fuming limits the pickling process to a room temperature process as increased temperatures cause even greater fuming. Room temperature operation results in a slower pickling rate. Moreover, because the pickling operation is exothermic, cooling equipment was required further adding to cost.
Attempts have been made in the prior art to reduce cost using hydrohalide acids by providing systems for the recovery of spent solutions. The recovery of hydrochloric acid from the spent pickling liquors used for the pickling of iron based metals is discussed in detail in articles by Perkins et al and Pool, Iron and Steel Engineer, April, 1965, pages 156 to 163, incorporated herein by reference. According to said publication, the high cost associated with the use of hydrochloric acid as a pickling solution is in part compensated for by the economy associated with the recovery system. Thus, the chloride salt formed during a pickling operation is recovered and decomposed thermally with almost complete recovery of the chloride ion which is used for the formation of reusable hydrochloric acid. This feature of the process enables hydrochloric acid to compete with sulfuric acid despite its high cost.
Even with the above described recovery process, the pickling operation is still expensive because of the excessive fuming of the acid during the pickling operation and the resultant loss thereof. The acid lost in this manner cannot be recovered and converted back to reusable hydrochloric acid.
In U.S. Pat. No. 3,423,240 incorporated herein by reference, there is taught a modification of the hydrochloric acid recovery system shown by Perkins et al which is stated to further improve the economy of the operation. According to said patent, the overall economy is improved by a modification comprising the use of extra chloride ion in the pickling bath in the form of a salt such as sodium chloride to compensate for losses in the system. The addition of the sodium chloride to the pickling solution accompanied by at least a stoichiometric amount of a sulphur containing substance capable of reacting with sodium chloride, oxygen and water vapor to produce hydrogen chloride and sodium sulphate is said to result in an almost quantitative conversion of the sodium chloride to hydrogen chloride. In plants operating both hydrochloric acid pickling lines and sulfuric acid pickling lines, the waste sulfuric acid pickling solution is an ideal source of the sulphur containing material. In situations where sulfuric acid pickling solutions are not readily available, recourse can be had to many other economical, commercially available sources of reactive sulphur.
Though the above procedure does make the overall process of pickling with hydrochloric acid and other hydrohalide acids more economical, it is still not as economical as might be desired because there is still an excessive loss of acid by fuming, thereby increasing the cost of the operation, attacking steel equipment and more importantly, presenting a health hazard due to attack on the respiratory system.